The Islamic State has obtained 90mm anti-tank rocket launchers like this one, shown in the hands last year of a Free Syria Army rebel. They were likely manufactured in the former Yugoslavia. (YouTube screen grab)

Anti-tank weapons that were likely once owned by moderate Syrian rebels have landed in the hands of Islamic State militants, according to a newly released field investigation conducted in both northern Iraq and Syria.

The Islamic State has also captured “significant quantities” of U.S.-manufactured small arms and has employed them on the battlefield, researchers found.

The investigation, led by a small-arms research organization known as Conflict Armament Research, marks a rare attempt to physically document the weapons being used by the Islamic State, the radical group that has expanded its control in parts of Syria and Iraq.

Militants with the group have picked up significant caches of arms after seizing Iraqi and Syrian military installations. The new research suggests they have also amassed arms after overrunning the moderate Syrian rebels being supplied by the United States and other allied nations.

To catalog the arms, field researchers embedded with Kurdish forces in Iraq and Syria for 10 days toward the end of July and were allowed access to Islamic State weapons that were captured after clashes. Along with the anti-tank weapons, manufactured in the former Yugoslavia, researchers documented a handful of U.S. M16A4 rifles, two Chinese Type 80 machine guns, a Croatian sniper rifle, a 9mm Glock pistol and various Soviet-era small arms.

In one case, U.S.-made weapons were found by the Kurdish forces near Ayn al-Arab, Syria. The weapons were likely obtained by the Islamic State after it conquered the northern Iraqi city of Mosul, roughly 300 miles away, according to field investigator Shawn Harris.

“They are transporting these weapons in batches, and have a solid organizational approach to moving these weapons around,” Harris said. “They’re operating as professionals.”

Additionally, the serial numbers on a number of the weapons recovered had been welded over, indicating that somewhere along their journey before they reached the Islamic State, a third party attempted to mask the weapons’ “chain of custody,” Harris said.

“It was clean,” Harris said, referring to the precision with which serial numbers were erased. “It indicates some foresight to our type of research.”

Researchers stressed that documenting weapons in the field can be a tenuous business and produces limited findings.

“We can only talk about what we see,” said another researcher, Damien Spleeters.

The most powerful weapons documented were the two 90mm Yugoslav anti-tank rocket launchers, known as “Osas,” which resembled rockets that were transferred to moderate Syrian rebels, reportedly by Saudi Arabia last year. It is purportedly shown in this video in the hands of a Free Syrian Army rebel last year:

The 25-pound rocket launchers have appeared in numerous battlefield videos shot in Syria and Iraq and were believed to be in Islamic State hands. But the new research marks the first time they have been physically documented, Spleeters said.

In March, the United States authorized the delivery of more sophisticated TOW anti-tank guided missiles to moderate rebels, after long resisting such deliveries out of fears that advanced weaponry would fall into the hands of extremist factions in Syria. Long before then, however, U.S. allies including Saudi Arabia and Qatar were funneling weaponry to various rebel factions in the conflict.

TOW missiles, which are wire-guided, are not among the weapons the Conflict Armament Research report documented.

“Not a single TOW anti-tank guided missile has fallen into the wrong hands,” said Oubai Shahbandar, a senior adviser to the Syrian Opposition Coalition. “The overwhelming majority of U.S. facilitated military aid is being used by the Free Syrian Army to combat ISIS in northern Syria. There is a thorough accountability process to ensure that allied military aid to Syrian freedom fighters is responsibly used in the field.”

Although captured weapons are of significant value to the Islamic State, their use hinges on a sufficient supply of the correct type of ammunition, noted Charles Lister, a visiting fellow at the Brookings Doha Center in Qatar.

For now, Lister said, the group does not seem to be facing any such shortages.

Fighters “have captured five major Syrian military bases in Syria since mid-July,” Lister said. “This will likely keep their momentum going for months to come.”

UPDATE, 12:00 p.m.: This report has been updated to clarify that the anti-tank weapons found in the possession of the Islamic State are not U.S.-made.